Dual substrate MEMS plate switch and method of manufacture

ABSTRACT

Systems and methods for forming an electrostatic MEMS plate switch include forming a deformable plate on a first substrate, forming the electrical contacts on a second substrate, and coupling the two substrates using a hermetic seal. The deformable plate may have at least one shunt bar located at a nodal line of a vibrational mode of the deformable plate, so that the shunt bar remains relatively stationary when the plate is vibrating in that vibrational mode. The hermetic seal may be a gold/indium alloy, formed by heating a layer of indium plated over a layer of gold. Electrical access to the electrostatic MEMS switch may be made by forming vias through the thickness of the second substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This U.S. patent application is related to U.S. patent application Ser.No. 11/211,623 (Attorney Docket No. IMT-Wallis), filed Aug. 26, 2005 andincorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Portions of the present invention were made with U.S. Government supportunder NSF SBIR Grant No. 0637474. The government may have certain rightsin this invention.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention relates to a microelectromechanical systems (MEMS) switchdevice, and its method of manufacture. More particularly, this inventionrelates to a MEMS electrostatic plate switch, which is manufactured ontwo separate substrates.

Microelectromechanical systems are devices often having moveablecomponents which are manufactured using lithographic fabricationprocesses developed for producing semiconductor electronic devices.Because the manufacturing processes are lithographic, MEMS devices maybe made in very small sizes, and in large quantities. MEMS techniqueshave been used to manufacture a wide variety of sensors and actuators,such as accelerometers and electrostatic cantilevers.

MEMS techniques have also been used to manufacture electrical relays orswitches of small size, generally using an electrostatic actuation meansto activate the switch. MEMS devices often make use ofsilicon-on-insulator (SOI) wafers, which are a relatively thick silicon“handle” wafer with a thin silicon dioxide insulating layer, followed bya relatively thin silicon “device” layer. In the MEMS devices, a thincantilevered beam of silicon may be etched into the silicon devicelayer, and a cavity is created adjacent to the thin beam, typically byetching the thin silicon dioxide layer below it to allow for theelectrostatic deflection of the beam. Electrodes provided above or belowthe beam may provide the voltage potential which produces the attractive(or repulsive) force to the cantilevered beam, causing it to deflectwithin the cavity.

One known embodiment of such an electrostatic relay is disclosed in U.S.Pat. No. 6,486,425 to Seki. The electrostatic relay described in thispatent includes a fixed substrate having a fixed terminal on its uppersurface and a moveable substrate having a moveable terminal on its lowersurface. Upon applying a voltage between the moveable electrode and thefixed electrode, the moveable substrate is attracted to the fixedsubstrate such that an electrode provided on the moveable substratecontacts another electrode provided on the fixed substrate to close themicrorelay.

However, to fabricate the microrelay described in U.S. Pat. No.6,486,425, the upper substrate must be moveable, so that the uppersubstrate must be thin enough such that the electrostatic force maycause it to deflect. The moveable substrate is formed from asilicon-on-insulator (SOI) wafer, wherein the moveable feature is formedin the silicon device layer, and the SOI wafer is then adhered to thefixed substrate. The silicon handle wafer and silicon dioxide insulatinglayer are then removed from the SOI wafer, leaving only the thin silicondevice layer which forms the moveable structure.

SUMMARY

Because the top substrate of the microrelay described in the '425 patentmust necessarily be thin enough to be moveable, it is also delicate andsusceptible to damage from contact during or after fabrication.

The systems and methods described here form an electrostatic MEMS plateswitch using dual substrates, a first, lower substrate on which to forma deformable plate with at least one electrical shunt bar to provide anelectrical connection between two contacts of a switch. These contactsmay be formed on a second, upper substrate. After forming thesestructures, the two substrates are bonded together to form the switch.It should be understood that the designation of “upper” and “lower” isarbitrary, that is, the deformable plate may also be formed on an uppersubstrate and the contacts may be formed on a lower substrate.

The electrostatic MEMS plate design may have a number of advantages overcantilevered switch designs. For example, in a plate design, thestiffness of the restoring force on the plate may no longer bedetermined by the plate dimensions, but instead may be determined byspring beams which support the deformable plate over the substratesurface. Therefore, weaker or stronger restoring forces may be usedwithout impacting the plate dimensions. This may allow the spacing ofthe contacts of the switch to be larger, or smaller, than that of thecantilever design without affecting the stiffness of the moveablestructure. Because the restoring force is provided by spring beams, thedevice may be made more compact than the cantilevered designs, which mayrequire a certain length of cantilevered beam to provide sufficientflexibility. Also, multiple switches may be placed on a singledeformable plate, whereas with the cantilevered design, only the area atthe distal end of the cantilevered beam is generally appropriate for theplacement of the switch.

Accordingly, in the systems and methods described here, the deformableplate is attached to the first SOI substrate by one or more narrowspring beams formed in the device layer of the SOI substrate. Thesespring beams remain fixed at their proximal ends to the silicon dioxideand handle layer of the SOI substrate. A portion of the silicon dioxidelayer adjacent to the deformable plate may be etched to release theplate, however, a silicon dioxide attachment point remains which couplesthe spring beams supporting the deformable plate to the silicon handlelayer. The silicon dioxide layer therefore provides the anchor point foradhesion of the deformable plate to the first, lower SOI substrate fromwhich it was made. Because the remainder of the rigid, SOI wafer remainsintact, it may provide protection for the switch against inadvertentcontact and shock.

Because the rigid SOI wafer remains intact, it may also be hermeticallybonded to a second, upper substrate at the end of the fabricationprocess. By forming the hermetic seal, the switch may enclose aparticular gas environment which may be chosen to increase the breakdownvoltage of the gas environment within the switch. Alternatively, theenvironment surrounding the plate switch may be vacuum, which mayincrease the switching speed of the plate switch by decreasing viscoussqueeze film damping which may arise in a gas environment. The hermeticseal may also protect the electrostatic MEMS switch from ambient dustand debris, which may otherwise interfere with the proper functioning ofthe device.

The deformable plate formed on the first substrate may carry one or moreshunt bars, placed at or near the nodal lines for a vibrational mode ofthe deformable plate. Points along these lines remain relativelystationary, even though the deformable plate may still be vibrating inthe vibrational mode. In one exemplary embodiment, the deformable platemay carry two shunt bars, each placed on a nodal line for a particularvibrational mode of the deformable plate known as the (2,0) mode,hereinafter called the third mode. This mode is well known to those wellversed in the art of plate modes. By placing the shunt bars in theselocations along nodal lines, the switch is relatively insensitive tocontinuing vibrations, and the switch may remain closed even when thedeformable plate is still moving.

In one exemplary embodiment, the deformable plate is coupled to thefirst, SOI substrate by four flexible spring beams which are anchored tothe dielectric layer of the SOI substrate at the proximal end of eachspring beam. The other end of the spring beams may be contiguous withthe deformable plate. The spring beams may include a substantiallyninety degree bend, so that each spring beam on one side of thedeformable plate extends in an opposite direction from the other. Thisembodiment may be referred to as the symmetric embodiment, as the twospring beams on each side of the deformable plate may have the sameshapes and orientations as the two spring beams on the other side of thedeformable plate. In another “asymmetric” embodiment, the spring beamson one side of the deformable plate may extend in one direction, and thespring beams on the other side of the deformable plate may extend in theopposite direction. The asymmetric embodiment may therefore be capableof twisting during vibration, which may provide a scrubbing action tothe deformable plate. The scrubbing action may clear contamination anddebris, thus reducing the contact resistance between the shunt bars onthe deformable plate and the contacts located on the second substrate.

In one exemplary embodiment, etch release holes may be placed betweenthe nodal lines of the deformable plate, so that the deformable platemay be made more flexible in critical regions. The etch release holesmay thereby encourage vibration in a particular vibrational mode overvibrations in other modes. In other exemplary embodiments, the etchrelease holes may be placed uniformly about the deformable plate in aclose-packed hexagonal array. This arrangement may reduce the mass ofthe deformable plate, and allow ambient gas to flow through the etchrelease holes and thus reducing squeeze film damping and increasing theswitching speed of the deformable plate.

In one exemplary embodiment, the method for manufacturing the MEMSswitch may include forming a deformable plate on a first substrate,forming at least one electrode on a second substrate, and coupling thefirst substrate to the second substrate with a seal that encloses theMEMS switch. By forming these features on separate substrates, thecleanliness of the contact points may be maintained during processing,before the substrates are sealed hermetically.

The seal may be hermetic, made by forming an alloy of gold and indium,AuIn_(x), where x is about 2. The alloy may be formed by melting a layerof indium deposited over a layer of gold. The hermetic seal is thereforealso conductive, and may provide electrical access to the deformableplate, for example. The hermetic seal may be particularly important forswitching applications involving relatively high voltage signals,wherein an insulating gas may be needed to prevent electrical breakdownof the environment between the high voltage electrodes. In such cases,the insulating gas, or vacuum, may need to be sealed hermetically tocreate an environment for the MEMS switch which can withstand highervoltages without breaking down, without allowing the gas to leak out of,or into, the MEMS switch seal.

In another exemplary embodiment, electrical access to the switch may begained using through hole vias formed through the second substrate. Byproviding electrical access through the second substrate, the hermeticseal may not be compromised by the presence of electrical leads beingrouted under the bond line.

The systems and methods described herein may be appropriate for thefabrication of an RF electrostatic MEMS plate switch which is capable ofoperating in the range of DC to 10 GHz.

These and other features and advantages are described in, or areapparent from, the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary details are described with reference to theaccompanying drawings, which however, should not be taken to limit theinvention to the specific embodiments shown but are for explanation andunderstanding only.

FIG. 1 is a cross sectional view of an exemplary dual substrateelectrostatic MEMS plate switch;

FIG. 2 is a greyscale image of the third vibrational mode of adeformable plate such as that used in the plate switch of FIG. 1;

FIG. 3 is a plan view of one exemplary embodiment of the deformableplate of the dual substrate electrostatic MEMS plate switch of FIG. 1,showing the locations of the two shunt bars along the nodal lines of thedeformable plate;

FIG. 4 is a greyscale image of the deformable plate in the thirdvibrational mode upon making contact with electrodes located below theshunt bars;

FIG. 5 is a plan view of a second exemplary embodiment of a deformableplate usable in the dual substrate MEMS plate switch of FIG. 1;

FIG. 6 is a plan view of a design for a third exemplary embodiment of adeformable plate usable in the dual substrate MEMS plate switch of FIG.1;

FIG. 7 is a diagram showing a first step in an exemplary method ofmanufacturing the first plate substrate of the dual substrate MEMS plateswitch of FIG. 1, using the deformable plate of FIG. 6;

FIG. 8 is a diagram showing a second step in an exemplary method ofmanufacturing the first plate substrate of the dual substrate MEMS plateswitch of FIG. 1, using the deformable plate of FIG. 6;

FIG. 9 is a diagram showing a third step in an exemplary method ofmanufacturing the first plate substrate of the dual substrate MEMS plateswitch of FIG. 1, using the deformable plate of FIG. 6;

FIG. 10 is a diagram showing a first step in an exemplary method ofmanufacturing the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 11 is a diagram showing a second step in an exemplary method ofmanufacturing the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 12 is a diagram showing a third step in an exemplary method ofmanufacturing the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 13 is a diagram showing a greater detail of the lower electrodeformed on the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 14 is a diagram showing the bonding pad design formed on thebackside of the dual substrate MEMS plate switch of FIG. 1;

FIG. 15 is a diagram of the completed dual substrate MEMS plate switchof FIG. 1, with an indication of the cross section shown in FIG. 16; and

FIG. 16 is a cross sectional view of the dual substrate MEMS plateswitch along the cross section indicated in FIG. 15.

DETAILED DESCRIPTION

In the systems and methods described here, an electrostatic MEMS switchis fabricated on two substrates. A deformable plate carrying at leastone shunt bar is formed on the first substrate, and the electricalcontacts of the switch, which will be connected via the shunt bar on thedeformable plate when the switch is closed, are formed on the othersubstrate. The two substrates may then be sealed hermetically by agold-indium seal. Electrical access to the switch may be afforded by aset of through hole vias, which extend through the thickness of thesecond substrate. Although the systems and methods are described asforming the deformable plate first on the first substrate followed bythe electrical contacts on the second substrate, it should be understoodthat this embodiment is exemplary only, and that the electrical contactsmay be formed first, or in parallel with, the formation of thedeformable plate.

FIG. 1 is a cross sectional view of the dual substrate electrostaticMEMS plate switch 100 fabricated on two substrates, a plate substrate1000 and a via substrate 2000. The plate substrate 1000 may be an SOIwafer, and the via substrate may be a silicon wafer. The switch 100 mayinclude a plate 1300 bearing at least one shunt bar 1100. The plate maybe deformable, meaning that it is sufficiently thin compared to itslength or its width to be deflected when a force is applied, and mayvibrate in response to an impact. For example, a deformable plate maydeflect by at least about 10 nm at its center by a force of about 1μNewton applied at the center, and sufficiently elastic to supportvibration in a plurality of vibrational modes. The deformable plate 1300may be suspended above the handle layer 1030 of an SOI substrate by fourspring beams (not shown in FIG. 1), which are themselves affixed to thehandle layer 1030 by anchor points formed from the dielectric layer 1020of the SOI plate substrate 1000. As used herein, the term “spring beam”should be understood to mean a beam of flexible material affixed to asubstrate at a proximal end, and formed in substantially one plane, butconfigured to move and provide a restoring force in a directionsubstantially perpendicular to that plane. The deformable plate maycarry at least one, and preferably two conductive shunt bars whichoperate to close the switch 100, as described below.

Each shunt bar is designed to span two contact points, 2110 and 2120,which are through wafer vias formed in the via substrate 2000, andcovered by a layer of contact material 2112 and 2122, respectively. Thedeformable plate may be actuated electrostatically by anadjacent-electrostatic-electrode 2300, which may be disposed directlyabove (or below) the deformable plate 1300, and may be fabricated on thevia substrate 2000. The deformable plate 1300 itself may form one plateof a parallel plate capacitor, with the electrostatic electrode 2300forming the other plate. When a differential voltage is placed on thedeformable plate 1300 relative to the adjacent electrostatic electrode2300, the deformable plate is drawn toward the adjacent electrostaticelectrode 2300. The action raises (or lowers) the shunt bar 1100 into aposition where it contacts the contact points 2110 and 2120, therebyclosing an electrical circuit. Although the embodiment illustrated inFIG. 1 shows the plate formed on the lower substrate and the vias andcontacts formed on the upper substrate, it should be understood that thedesignation “upper” and “lower” is arbitrary. The deformable plate maybe formed on either the upper substrate or lower substrate, and the viasand contacts formed on the other substrate. However, for the purposes ofthe description which follows, the embodiment shown in FIG. 1 ispresented as an example, wherein the plate is formed on the lowersubstrate and is pulled upward by the adjacent electrode formed on theupper substrate.

FIG. 2 is a greyscale image of a thin, deformable plate in a vibrationalmode. The image was generated by a finite element model, using platedimensions of 200 μm width by 300 μm length by 5 μm thickness. Thedeformable plate is supported by four spring beams 10 μm wide and 5 μmthick, extending from two sides of the deformable plate. According tothe model, a first vibrational mode with a frequency of 73 kHz may besimply the movement of the entire plate, substantially undeflected,toward and away from the surface to which it is attached by the springbeams. A second vibrational mode with a frequency of 171 kHz occurs whenthe deformable plate twists about its long axis, by bending at thejoints between the deformable plate and the spring beams.

However, another vibrational mode exists as illustrated by FIG. 2, whichis encouraged by the proper placement of the spring beams. The springbeams are placed at approximately the location of the node lines forthis vibrational mode. By placing the spring beams at these points, theplate may vibrate with relatively little deflection of the spring beams.The frequency associated with this mode is at about 294 kHz.

As a result, the deformable plate vibrates substantially in the thirdvibrational mode, with the node lines of the vibration locatedsubstantially at the locations of the supporting spring beams. Thesenode lines indicate points on the deformable plate which remainrelatively stationary, compared to the ends and central region which aredeflected during the vibration. The existence of these node linesindicate advantageous locations for the placement of electrodes for aswitch, because even when the plate is vibrating, there is relativelylittle deflection of the plate along the node lines. Accordingly, if ashunt bar is placed at the node lines, the shunt bar may provideelectrical conductivity between two electrodes located beneath the shuntbar, even if the plate continues to vibrate.

FIG. 3 is a plan view of a first exemplary embodiment of a deformableplate useable in the plate switch of FIG. 1. The plate is supported byfour spring beams 1330, which are attached to the underlying substrateat their proximal ends 1335. One pair of the four spring beams may bedisposed on one side of the deformable plate, and another pair of thefour spring beams may be disposed on an opposite side of the deformableplate. Each spring beam may have a segment extending from the deformableplate which is coupled to an adjoining segment by a bend. The choice ofangle for this bend may affect the kinematics of the deformable plate1300.

In the embodiment shown in FIG. 3, the spring beams include a ninetydegree bend, such that each spring beam on each side of the deformableplate 1300 extends in an opposite direction to the adjacent spring beam.This embodiment may be referred to as the symmetric embodiment, as theorientation of the deformable plate and spring beams is symmetric withrespect to reflection across either a longitudinal or latitudinal axisof the deformable plate, wherein the longitudinal or latitudinal axis isdefined as horizontal or vertical line, respectively, passing throughthe center of the deformable plate. It should be understood that thisembodiment is exemplary only, and that the spring beams may bend withother angles, for example, twenty or thirty degrees, rather than ninetyas shown in FIG. 3.

The two nodal lines for the third vibrational mode are shown in FIG. 3.One of two shunt bars 1110 and 1120 may be placed across each nodalline. The shunt bars 1110 and 1120 may be electrically isolated from thedeformable plate by a layer of dielectric 1210 and 1220, respectively.Additional dielectric standoffs 1230 may be formed at the corners ofdeformable plate 1300, to prevent deformable plate 1300 from contactingthe adjacent electrostatic electrode 2300 at the corners of deformableplate 1300, when actuated by the adjacent electrostatic electrode 2300.The shunt bars 1110 and 1120 may be dimensioned appropriately to spanthe distance between two underlying electrical contacts, 2110 and 2120under shunt bar 1110, and contacts 2210 and 2220 under shunt bar 1120.The deformable plate 1300 is actuated when a voltage differential isapplied to an adjacent electrode, which pulls the deformable plate 1300toward the adjacent electrode. If the deformable plate 1300 vibrates asa result of actuation, it is likely to vibrate in the third vibrationalmode shown in FIG. 2. Accordingly, the shunt bars 1110 and 1120 areplaced advantageously at the nodal lines of this vibrational mode.

The tendency of deformable plate 1300 to vibrate in the thirdvibrational mode may be enhanced by placing etch release holes 1320along the latitudinal axis passing through the center of the deformableplate, between the nodal lines, as shown in FIG. 3. These etch releaseholes are used to assist the liquid etchant in accessing the farrecessed regions beneath the deformable plate, to remove the dielectriclayer beneath the plate, as described further in the exemplarymanufacturing process set forth below. By placing these etch releaseholes appropriately, the deformable plate 1300 may be made more flexiblein certain regions, such as along the latitudinal axis, such that theplate is encouraged to vibrate in a mode such that the maximumdeflection occurs where the plate is more flexible. For example, toencourage the vibration as shown in FIG. 2, the plate may be made moreflexible along its latitudinal axis, in order to accommodate the regionsundergoing the maximum deflection, by placing etch release holes 1320along this latitudinal axis.

In another alternative embodiment, the etch release holes are disposedin a close-packed hexagonal array over the entire surface of thedeformable plate 1300. Such an embodiment may be advantageous in thatthe mass of the deformable plate is reduced, and multiple pathways areprovided for the flow of the ambient gas to either side of thedeformable plate. Both of these effects may improve the switching speedof the device by reducing the inertia of the deformable plate 1300 andreducing the effects of squeeze film damping.

FIG. 4 is a greyscale image of the deformable plate shown in FIG. 3,after actuation by an adjacent electrode, calculated by a finite elementmodel. As shown in FIG. 4, the deformable plate is pulled down towardthe adjacent electrode, which in this case is located beneath thedeformable plate 1300. The lowest areas of the deformed plate are in thevicinity of the contacts, also located beneath the deformable plate1300. When the deformable plate is deflected as shown in FIG. 4, theshunt bars affixed to the deformable plate are lowered onto theunderlying contacts, thus providing a conductive path between thecontacts and closing the switch 100. Any residual vibration in thedeformable plate is primarily in the third vibrational mode, depicted inFIG. 2. Thus, for shunt bars placed as shown in FIG. 3, the residualvibration does not substantially affect the ability of switch 100 toclose the conductive path between the contacts.

Also as shown in FIG. 4, the corners of deformable plate 1300 tend to bedrawn towards the adjacent actuation electrode. The dielectric standoffs1230 may prevent the touching of corners of the deformable plate 1300 tothe adjacent electrode, thus shorting the actuation voltage. Theactuation voltage in this simulation is about 40 volts, and the size ofthe deformable plate is about 200 μm by 300 μm. This actuation voltageproduces a deflection of at least about 0.6 μm in the deformable plate.This deflection is about ⅓ of the overall separation between the shuntbars and the electrodes, which may nominally be about 2.5 μm, and issufficient to cause snap-down of the deformable plate onto theunderlying contacts. Although as shown in FIG. 4, the maximum deflectionis near the center of the plate, this effect may be altered by disposingthe spring beams at an angle shallower than ninety degrees. Such anarrangement may result in a more consistent force being applied betweenthe shunt bar and each of the underlying contacts.

FIG. 5 is a plan view of a second exemplary embodiment of the deformableplate 1300′. Deformable plate 1300′ may differ from deformable plate1300 by the placement and orientation of the four spring beams whichsupport the deformable plate 1300′. A first set of spring beams 1332′are coupled to one side of the deformable plate 1300′, and a second setof spring beams 1330′ are coupled to the other side of deformable plate1300′. However, in contrast to the spring beams 1330 of deformable plate1300, spring beams 1330′ extend in an opposite direction to spring beams1332′ of deformable plate 1300′, after the bend in spring beams 1330′and 1332′. This may allow deformable plate 1300′ to twist and translatesomewhat in the plane of the deformable plate 1300′, upon actuation byapplying a differential voltage between deformable plate 1300′ and anadjacent electrode, because of the flexibility of the bend between thebeam segments. This twisting action may allow some lateral movement ofshunt bars 1100 over contacts 2110 and 2120, thereby scrubbing thesurface of the contacts to an extent. This scrubbing action may removecontamination and debris from the contact surfaces, thereby allowingimproved contact and lower contact resistance.

The embodiment shown in FIG. 5 may be referred to as the anti-symmetricembodiment, because the spring beams 1330′ extend from the beam in anopposite direction compared to spring beams 1332′. In other words, inthe anti-symmetric embodiment 1300′, the beam springs disposed on oneside of the deformable plate are anti-symmetric with respect to thebeams springs disposed on the opposite side of the deformable plate.Thus, when deformable plate 1300′ is reflected across a longitudinal orlatitudinal axis, the spring beams extend in an opposite direction fromthe bend. It should be understood that although a ninety-degree bend isillustrated in FIG. 5, the bend may have angles other thanninety-degrees, for example, for example, twenty or thirty degrees.

As shown in FIGS. 3 and 5, the deformable plate 1300 may have two shuntbars 1100 placed upon dielectric isolation layers 1100. Each shunt barmay close a respective set of contacts. For example, shunt bar 1110 inFIG. 3 may close one set of contacts 2210 and 2220, whereas shunt bar1120 may close a second set of contacts 2110 and 2120. Therefore, eachdual substrate MEMS plate switch may actually have two sets of switchcontacts disposed in parallel with one another. The dual substrate MEMSplate switch may therefore still operate if one set of switch contactsfails. Furthermore, the overall switch resistance is only one-half ofthe switch resistance that would exist with a single set of switchcontacts, because the two switches are arranged in parallel with oneanother.

FIG. 6 is a plan view of a layout of deformable plate 1300, showingadditional detail of the embodiment. In particular, spring beams 1330are formed with cutouts 1350 which penetrate the deformable plate 1300.The deformable plate may also have relieved areas 1340 formed near thelocations of the shunt bars 1100. Both the cutouts 1350 and the relievedareas 1340 give the deformable plate additional flexibility in the areaof the junction with the spring beams 1330. This may help decouple themotion of the plate 1300 from the deflection of the spring beams 1330.These features 1350 and 1340 may also help the deformable plate 1300 toclose the switch effectively, in the event that the contacts 2210 and2220 are recessed somewhat from the surface of the via substrate 2000,by giving the deformable plate 1300 additional flexibility in the regionaround the shunt bars 1200.

As shown in FIG. 6, deformable plate 1300 may have a length of about 300μm and a width of about 200 μm. The separation d1 between the springbeams may be about 130 μm. The lengths of each segment d2 and d3 of thespring beams 1330 may be about 100 μm, so that the total length of thespring beams 1330 is about 200 μm. The lengths d4 of the cutouts 1350may be about 50 μm, or about half the length of the beam segment d3. Thewidth of the spring beam 1330 may be about 12 μm. The distance betweenthe relieved areas 1340 may also be about 100 μm. The dimensions of theshunt bars 1100 may be about 40 μm width and about 60 μm length. Thediameter of the via contacts 2110 and 2120 may be about 30 μm to about50 μm. It should be understood that these dimensions are exemplary only,and that other dimensions and designs may be chosen depending on therequirements of the application.

Since the deformable plate 1300 may be made from the device layer 1010of the SOI plate substrate 1000, it may be made highly resistive, of theorder 20 ohm-cm. This resistivity may be sufficient to carry theactuation voltage of about 40 volts, but may too high to support thehigher frequency alternating current voltages associated with the firstvibrational mode at about 73 kHz. Accordingly, the resistivity mayelectrically dampen capacitive plate vibrations, especially thewhole-body first mode plate vibration.

The electrostatic plate switch design illustrated in FIG. 6 may have anumber of advantages over cantilevered switch designs, wherein theswitch contacts are disposed at the end of a cantilevered beam. Forexample, as described above, multiple sets of switch contacts may beprovided along a deformable plate, thereby reducing the overall switchresistance and therefore the loss across the switch. The multiple switchcontacts also provide redundancy, such that the switch may still beuseable even if one set of switch contacts fails. These design optionsare generally not available in a cantilevered switch design, because thecontacts are necessarily placed at the distal end of the cantileveredbeam.

In addition, the electrostatic deformable plate switch 100 may be mademore compact than a cantilevered switch, because a long length ofcantilevered beam is not required to have a sufficiently flexible memberto actuate with modest voltages. For example, the plate designillustrated in FIG. 6 may be actuated with only 40 volts, because thespring beams 1330 which support the deformable plate may be maderelatively flexible, without impacting the spacing between theelectrical contacts 2110 and 2120.

Because the restoring force of the switch is determined by the springbeam 1330 geometry, rather than the plate 1300 geometry, modificationsmay be made to the plate 1300 design without affecting the kinematics ofthe spring beams 1330. For example, as mentioned above, a plurality ofetch release holes 1310 may be formed in the deformable plate 1300,without affecting the stiffness of the restoring spring beams 1330.These release holes 1310 may allow air or gas to transit readily fromone side of the deformable plate 1300 to the other side, therebyreducing the effects of squeeze film damping, which would otherwisereduce the speed of the device. These etch release holes 1310 may alsoreduce the mass of the deformable plate 1300, also improving itsswitching speed, without affecting the restoring force acting on thedeformable plate 1300 through the spring beams 1330.

By placing the shunt bars near the nodal lines of a vibrational mode,the switching speed may be improved because the shunt contact interfereswith vibratory motion in other modes. This effectively damps thevibrations in other modes. By placing the shunt bars at the nodal linesof a vibrational mode, the movement of the shunt bar is minimal, even ifthe plate is still vibrating in this mode. Therefore, although thedeformable plate may be made exceptionally light and fast because of itssmall size and plurality of etch release holes, it vibrates onlyminimally because of its damping attributes. Accordingly, theelectrostatic MEMS plate switch illustrated in FIG. 6 may be used in avacuum environment, which is often not possible because in a vacuum,vibrations are no longer damped by viscous air motion around the movingmember of the switch.

Because through wafer vias are used to route the signal to and from thedual substrate electrostatic MEMS plate switch 100, the switch 100 maybe particularly suited to handling high frequency, RF signals. Withoutthe through wafer vias, the signal would have to be routed along thesurface of the second via substrate 2000, and under the hermetic bondline. However, because the hermetic bond line is metallic and grounded,this allows substantial capacitive coupling to occur between the surfacerouted signal lines and the ground plane of the device, which liesdirectly adjacent to, and narrowly separated from the signal lines inthe bonding area. The through wafer vias allow this geometry to beavoided, thus reducing capacitive coupling and substantially improvingthe bandwidth of the device. The through wafer vias may also act as heatsinks, leading the heat generated in the switch to be directed quicklyto the opposite side of the wafer and to the large bonding pads 2115 and2125 on the backside of the device for dissipation.

FIGS. 7-15 depict steps in an exemplary method for manufacturing thedual substrate MEMS plate switch 100. The steps are divided into threesections: those steps depicted in FIGS. 7-9 pertaining to thepreparation of features on the plate substrate 1000; those stepsdepicted in FIGS. 10-12 pertaining to the preparation of features on thevia substrate 2000; those steps depicted in FIGS. 14-15 pertaining tothe bonding to the plate substrate 1000 to the via substrate 2000, andformation of bond pads on the backside of the via substrate, to completethe device. FIG. 16 shows a cross section of the completed device shownin plan view in FIG. 15.

FIG. 7 depicts a first step in an exemplary method for manufacturing thefeatures on the plate substrate 1000 shown in FIG. 1. The platesubstrate 1000 may be a silicon-on-insulator substrate including a 5 μmthick device layer 1010, a 2 μm thick buried dielectric layer 1020, anda 500 μm thick handle layer 1030. In one exemplary embodiment, theburied dielectric layer may be a layer of silicon dioxide, and the stepsdescribed below are appropriate for this embodiment. The first step mayinclude the formation of etch release holes 1310 in the device layer1010 of the SOI plate substrate 1000. These holes 1310 may be formed bydepositing and patterning photoresist in the appropriate areas, and dryetching the release holes through the device layer 1010, using thedielectric layer 1020 as an etch stop. These release holes 1310 may be,for example, about 2 μm to about 10 μm in diameter. If the release holesare distributed over the surface of the deformable plate to reduce themass of the plate and improve mode coupling, they may be arranged in ahexagonal, close-packed array with diameters of 2 μm and spaced 3 μmapart. Deep reactive ion etching (DRIE) may be performed to etch therelease holes using, for example, an etching tool manufactured bySurface Technology Systems of Newport, UK. Such a tool may be used forthis and later DRIE steps, described below.

After etching the release holes 1310 in the device layer 1030, a thinmultilayer of 15 nm chromium (Cr) and 100 nm nickel (Ni) may besputtered onto the backside of the plate substrate 1000, for use as aplating base for the plating of a thicker layer of protective material,such as copper (Cu) or nickel (Ni). This protective layer of copper ornickel may protect the native oxide 1040 existing on the backside of thehandle layer 1030 of the SOI substrate during the hydrofluoric acid etchto follow. The protective layer of copper or nickel may be about 4 μmthick, and may also minimize the wafer bow during further processing.

The dielectric layer 1020 may then be etched away beneath and around theetch release holes 1310, using a hydrofluoric acid liquid etchant, forexample. The liquid etch may remove the silicon dioxide dielectric layer1020 in all areas where the deformable plate 1300 is to be formed. Theliquid etch may be timed, to avoid etching areas that are required toaffix the spring beams 1330 of the deformable plate 1300, which will beformed later, to the handle layer 1030. Additional details as to the dryand liquid etching procedure used in this method may be found in U.S.patent application Ser. No. 11/359,558 (Attorney Docket No. IMT-SOIRelease), incorporated by reference in its entirety.

The next step in the exemplary method is the formation of the dielectricpads 1200, 1210, and 1220, and dielectric standoffs 1230 as depicted inFIG. 3. Pad structures 1200, 1210 and 1220 form an electrical isolationbarrier between the shunt bar 1100 and the deformable plate 1300,whereas standoffs 1230 form a dielectric barrier preventing the cornersof the deformable plate 1300 from touching the adjacent actuationelectrode 2300. The deformable plate 1300 and adjacent actuationelectrode 2300 form the two plates of a parallel plate capacitor, suchthat a force exists between the plates when a differential voltage isapplied to them, drawing the deformable plate 1300 towards the adjacentactuation electrode 2300.

The dielectric structures 1200, 1210, 1220 and 1230 may be silicondioxide, which may be sputter-deposited over the surface of the devicelayer 1010 of the SOI plate substrate 1000. The silicon dioxide layermay be deposited to a depth of, for example, about 300 nm. The 300 nmlayer of silicon dioxide may then be covered with photoresist which isthen patterned. The silicon dioxide layer is then etched to formstructures 1200, 1210, 1220 and 1230. The photoresist is then removedfrom the surface of the device layer 1010 of the SOI plate substrate1000. Because the photoresist patterning techniques are well known inthe art, they are not explicitly depicted or described in furtherdetail.

FIG. 8 depicts a second step in the preparation of the SOI plate wafer1000. In the second step, a conductive material is deposited andpatterned to form the shunt bar 1200 and a portion of what will form thehermetic seal. The hermetic seal may include a metal alloy formed frommelting a first metal into a second metal, and forming an alloy of thetwo metals which blocks the transmission of gases. In preparation offorming the hermetic seal, a perimeter of the first metal material 1400may be formed around the deformable plate 1300. The conductive materialmay actually be a multilayer comprising first a thin layer of chromium(Cr) for adhesion to the silicon and/or silicon dioxide surfaces. The Crlayer may be from about 5 nm to about 20 nm in thickness. The Cr layermay be followed by a thicker layer about 300 nm to about 700 nm of gold(Au), as the conductive metallization layer. Preferably, the Cr layer isabout 15 nm thick, and the gold layer is about 600 nm thick. Anotherthin layer of molybdenum may also be used between the chromium and thegold to prevent diffusion of the chromium into the gold, which mightotherwise raise the resistivity of the gold.

Each of the Cr and Au layers may be sputter-deposited using, forexample, an ion beam deposition chamber (IBD). The conductive materialmay be deposited in the region corresponding to the shunt bar 1100, andalso the regions which will correspond to the bond line 1400 between theplate substrate 1000 and the via substrate 2000 of the dual substrateelectrostatic MEMS plate switch 100. This bond line area 1400 ofmetallization will form, along with a layer of indium, a seal which willhermetically seal the plate substrate 1000 with the via substrate 2000,as will be described further below.

While a Cr/Au multilayer is disclosed as being usable for themetallization layer of the shunt bar 1100, it should be understood thatthis multilayer is exemplary only, and that any other choice ofconductive materials or multilayers having suitable electronic transportproperties may be used in place of the Cr/Au multilayer disclosed here.For example, other materials, such as titanium (Ti) may be used as anadhesion layer between the Si and the Au. Other exotic materials, suchas ruthenium (Ru) or palladium (Pd) can be deposited on top of the Au toimprove the switch contact properties, etc. However, the choicedescribed above may be advantageous in that it can also participate inthe sealing of the device through the alloy bond, as will be describedmore fully below.

FIG. 9 shows the plate substrate 1000 of the dual substrateelectrostatic MEMS plate switch 100 after the silicon device layer hasbeen patterned to form the deformable plate 1300. To form the deformableplate, the surface of the device layer 1010 of the SOI plate substrate1000 is covered with photoresist which is patterned with the design ofthe deformable plate. The deformable plate outline is the etched intothe surface of the device layer by, for example, deep reactive ionetching (DRIE). Since the underlying dielectric layer 1020 has alreadybeen etched away, there are no stiction issues arising from the liquidetchant, and the deformable plate is free to move upon its formation byDRIE. As before, since the photoresist deposition and patterningtechniques are well known, they are not further described here.

Preparation of the plate substrate 1000 is thereby completed. Thedescription now turns to the fabrication of the via substrate 2000, asillustrated in FIGS. 10-12.

FIG. 10 shows a first step in fabricating the via substrate 2000 of thedual substrate electrostatic MEMS plate switch 100. The via substrate2000 may be, for example, silicon, glass, or any other suitable materialconsistent with the process described below, or suitable equivalentsteps. In one exemplary embodiment, the via substrate is a 500 μm thicksilicon wafer. The via substrate 2000 may be covered with a photoresist,which is patterned in areas corresponding to the locations of vias 2110,2120, 2210, 2220, 2400 and 2450, or electrical conduits that will beformed in the via substrate 2000.

Blind trenches may first be etched in the substrate 2000, for theformation of a set of vias 2110, 2120, 2210, 2220, 2400 and 2450 whichwill be formed in the trenches by plating copper into the trenches. A“blind trench” is a hole or depression that does not penetrate throughthe thickness of the via substrate 2000, but instead ends in a dead endwall within the material. The etching process may be reactive ionetching (RIE) or deep reactive ion etching (DRIE), for example, whichmay form blind trenches, each with a dead-end wall. The etching processmay be timed to ensure that the vias 2110, 2120, 2210, 2220, 2400 and2450 extend substantially into the thickness of the via substrate. Forexample, the vias 2110, 2120, 2210, 2220, 2400 and 2450 may be etched toa depth of about 80 μm to about 150 μm deep into the via substrate 2000.When the vias are completed as described below, via 2450 may provideelectrical access to the deformable plate 1300, and provide a voltagefor one side of the parallel plate capacitor which may provide theelectrostatic force required to close the switch; via 2400 may provideelectrical access to the electrostatic plate 2300 which forms the otherside of the parallel plate capacitor; via 2110 may provide electricalaccess to one of the contact electrodes 2112 of the switch; via 2120 mayprovide electrical access to the other contact electrode 2122 of theswitch, and so forth. After etching the blind trenches 2100-2450, thevia substrate may be cleaned with a solvent to remove any polymers thatmay remain on the walls of the blind trenches after the dry etchprocedure.

After formation of the blind trenches 2100-2450 and cleaning thereof,the substrate 2000 may be allowed to oxidize thermally, to form a layerof silicon dioxide 2050, which electrically isolates one via from thenext, as shown in FIG. 1. The oxide may be about 2 μm thick, forexample. A seed layer (not shown) may then be deposited on the uppersurface and in the blind trenches. The seed layer may be, for example, athin layer of chromium followed by a thin layer of gold, the chromiumfor adhesion and the gold as a seed layer for the plating of copper intothe vias 2110-2450. The chromium/gold seed layer may be, for example,about 850 nm in thickness, with about 100 nm of chromium and about 750nm of gold, and may be deposited by, for example, ion beam deposition(IBD) to provide an electrically continuous film of plating base to thebottom and sides of the vias. Metals, such as Cu, may also be depositedusing chemical vapor deposition (CVD) methods, so long as the metal is acompatible seed layer for the conductive material to be subsequentlyplated into the blind trench.

In order to fill the blind trenches 2100-2450 completely with theconductive material, the seed layer may be plated usingreverse-pulse-plating, as described in more detail in co-pending U.S.patent application Ser. No. 11/482,944 (Attorney Docket No. IMT-RPPVias), incorporated by reference-herein in its-entirety.

The blind trenches 2110-2450 may then be plated with copper, forexample, or any other suitable conductive material that can be platedinto the blind trenches, such as gold (Au) or nickel (Ni), to createvias 2110-2450. To assure a complete fill, the plating process may beperformed until the plated material fills the blind trenches to a pointup and over the surface of the substrate 2000. The surface of thesubstrate 2000 may then be planarized, using, for example, chemicalmechanical planarization, until the plated vias 2110-2450 are flush withthe surface of the substrate 2000, as shown in FIG. 10. Theplanarization process may stop on the seed layer or the dielectric layer2050 of the substrate, leaving for example, about 1 μm of the previouslygrown dielectric layer 2050, which continues to provide electricalisolation between the interior metal structures of the devices, whichwould otherwise be electrically connected by the silicon via substrate2000.

A standoff 2500 may then be formed on the substrate 2000, as shown inFIG. 10. This standoff may determine the separation between the platesubstrate 1000 bearing the deformable plate 1300 and the via substrate2000, when the two substrates are bonded together. Any mechanicallyrigid material may be used, which is capable of forming a sufficientlystiff standoff. In one convenient embodiment, a polymer such asphotoresist is patterned and cured for use as standoffs 2510 and 2520.The polymer may be, for example, about 1 μm in thickness. Thephotoresist may be deposited and patterned, after which the remainingphotoresist portions 2500 may be baked to completely cure thesestructures. The negative tone photoresist SU-8, developed by IBM ofArmonk, N.Y., may be a suitable material for forming the standoff 2500.

Another metallization layer is then deposited over the substrate 2000,as shown in FIG. 11, which will form the bond ring 2600 as well ascontact electrodes 2112, 2122, 2212 and 2222. Metallization region 2300is also deposited in this step, which will form the adjacent electrodein the parallel plate capacitor of the switch. In one exemplaryembodiment, the metallization layer may actually be a multilayer ofCr/Au, the same multilayer as was used for the metallization layer 1400on the plate substrate 1000 of the dual substrate electrostatic MEMSplate switch 100. The metallization multilayer may have similarthicknesses and may be deposited using a similar process as that used todeposit metallization layer 1400 on substrate 1000. The metallizationlayer may also serve as a seed layer for the deposition of indium, asdescribed below.

Although metallization layer is described as consisting of a thinadhesion layer of Cr, and an optional antidiffusion layer of Mo,followed by a relatively thick layer of Au, it should be understood thatthis embodiment is exemplary only, and that any material havingacceptable electrical transport characteristics may be used asmetallization layer 2600. In particular, additional exotic materials maybe deposited over the gold, to achieve particular contact properties,such as low contact resistance and improved wear.

Photoresist may then be deposited on metallization layer, and patternedto provide features needed to form contacts 2112, 2122, 2212, 2222, 2300and 2600. The photoresist is exposed and developed to correspond toregions 2100-2300 and 2600. The substrate with the Cr/Au conductivematerial may then be wet etched to produce the conductive features2100-2300 and 2600. A suitable wet etchant may be iodine/iodide for theAu and permanganate for the Cr. FIG. 13 shows greater detail of contacts2112, 2122, 2212, 2222, 2300 and 2400. Also shown in FIG. 13 arefeatures 2330, which serve as regions in which the gold electrode can beelectrically isolated from the gold which comes into contact with thedielectric standoffs 1230 when the switch is closed.

Photoresist may then again be deposited over metallization layer 2600,and patterned to provide features for the plating of an indium layer2700, as shown in FIG. 12. The indium layer 2700 will, along with the Aulayer, form a hermetic seal that will bond the plate substrate 1000 tothe via substrate 2000 of dual substrate electrostatic MEMS plate switch100. The substrate 2000 with the patterned photoresist layer may then beimmersed in an indium plating bath, such that indium layers 2700 areplated in the feature, as shown in FIG. 12. The thickness of the platedindium layer may be, for example, about 3 μm to about 6 μm, and morepreferably about 4 μm. It may be important to control the relativethickness (and therefore volume) of the indium compared to the thicknessof the Au in metallization layer 2600, such that the ratio of materialsmay be appropriate to form an alloy of stoichiometry AuIn_(x), where xis about 2. Since the molar volume of indium is about 50% greater thangold, a combined gold thickness of both wafers of about 800 nm to about1600 nm may be approximately correct to form the AuIn₂ alloy. It mayalso be important to provide sufficient gold thickness that a thin layerof gold remains on the surface of the substrate 2100 to provide goodadhesion to the substrate, after the formation of the gold/indium alloy.This can additionally be ensured by plating the indium layer narrowerthan the gold metallization layers, as shown in FIG. 12, such that thefinal volumes and ratio of gold/indium provides for a slight excess ofgold at the substrate interface.

It may be important for gold metallization 2600 be wider in extent thanthe plated indium layer 2700. The excess area may allow the indium toflow outward somewhat upon melting, without escaping the bond region,while simultaneously providing for the necessary Au/In ratios citedabove.

The two portions, the plate substrate 1000 and the via substrate 2000are now ready to be assembled to form the dual substrate electrostaticMEMS plate switch 1100. The two portions may be first aligned, such thatthe metallization layers 1400 of plate substrate 1600 are registeredwith the metallization layers 2700 of the via substrate 2000. Thisplaces the plated indium layer 2700 between gold metallization layers1400 and 2600.

Methods and techniques for forming the alloy seal are further describedin U.S. patent application Ser. No. 11/211,625 (Attorney Docket No.IMT-Interconnect) and U.S. patent application Ser. No. 11/211,622(Attorney Docket No. IMT-Preform), each of which is incorporated byreference herein in its entirety.

For MEMS switches that benefit from a defined ambient environment, thetwo portions 1000 and 2000 of the electrostatic MEMS plate switch 100may first be placed in a chamber which is evacuated and then filled withthe desired gas. For example, for MEMS switches to be used in telephoneapplications using relatively high voltage signals, the desired gas maybe an insulating gas such as sulfur hexafluoride (SF₆), CO₂ or a freonsuch as CCl₂F₂ or C₂Cl₂F₄. The insulating gas may then be sealed withinthe dual substrate electrostatic MEMS plate switch 100 by sealing theplate substrate 1000 with the via substrate 2000 with the alloy bondformed by layers 1400, 2600 and 2700. Alternatively, an evacuated orsub-ambient or super-ambient environment may be sealed in the MEMS plateswitch 100 with a substantially hermetic seal. The term “substantiallyhermetic” may be understood to mean that the environment sealed with thedevice at manufacture retains at least about 90% of its originalcomposition over the lifetime of the device. For a device sealed with asub-ambient or super-ambient environment, the pressure at itsend-of-life may be within about 10% of its pressure at manufacture.

To form the alloy bond between layers 1400, 2600 and 2700, platesubstrate 1000 may be applied to the via substrate 2000 under pressureand at elevated temperature. For example, the pressure applied betweenthe cantilevered portion 1000 and the electrical contacts portion 2000may be from 0.5 to 2.0 atmospheres, and at an elevated temperature ofabout 180 degrees centigrade. This temperature exceeds the melting pointof the indium (157 degrees centigrade), such that the indium flows intoand forms an alloy with the gold. As mentioned above, the stoichiometryof the alloy may be about 2 indium atoms per one gold atom, to formAuIn_(x) where x is about 2. In contrast to the low melting point of theindium metal, the melting point of the alloy is 541 degrees centigrade.Therefore, although the alloy is formed at a relatively low temperature,the durability of the alloy bond is outstanding even at several hundreddegrees centigrade. The bond is therefore compatible with processeswhich deposit vulnerable materials, such as metals, on the surfaces andin the devices. These vulnerable materials may not be able to survivetemperatures in excess of about 200 degrees centigrade, withoutvolatilizing or evaporating.

Upon exceeding the melting point of the indium, the indium layers 2700flows outward, and the plate substrate 1000 and the via substrate 2000are pushed together, until their approach is stopped by the polymerstandoff 2500. As the alloy forms, it may immediately solidify, sealingthe preferred environment in the dual substrate electrostatic MEMS plateswitch 100.

While the systems and methods described here use a gold/indium alloy toseal the MEMS plate switch, it should be understood that the dualsubstrate electrostatic MEMS plate switch 100 may use any of a number ofalternative sealing methodologies, including different constituentmetals for the bond-line and cross-linked polymers. For example, theseal may also be formed using a low-outgassing epoxy which isimpermeable to the insulating gas.

In order to apply the appropriate signals to contact pads 2112, 2122,2212, 2222, 2400 and 2450, electrical access may need to be achieved tovias 2112, 2122, 2212, 2222, 2400 and 2450. As described earlier, vias2110, 2120, 2210, 2220, 2400 and 2450 begin as blind trenches formed inone side of the substrate, and plated with a conducting material. Toprovide access the conducting vias formed in the front side, materialfrom the opposite, back side of substrate 2000 may be removed until thedead-end walls of the blind trenches 2110-2450 have been removed, suchthat electrical access to the vias may be made from the back side. Inone exemplary embodiment, the original 500 μm thick silicon wafer isbackground until it has a thickness of about 80 μm, and the vias 2110,2120, 2210, 2220, 2400 and 2450 extend through the entire thickness ofthe remaining silicon. The technique for removing the excess materialmay be, for example, grinding. The processes used to form the vias isdescribed in more detail in U.S. patent application Ser. No. 11/211,624(Attorney Docket No. IMT-Blind Trench) and U.S. patent application Ser.No. 11/482,944 (Attorney Docket No. IMT-RPP Vias), incorporated byreference herein in their entireties.

The via substrate 2000 may then be coated with an oxide 2200, which maybe SiO₂, for example, at a thickness sufficient to isolate the vias2110-2220 one from the other. The oxide may be deposited by a lowtemperature dielectric deposition process, such as sputtering or plasmaenhanced chemical vapor deposition (PECVD) to a thickness of about 1 μm.The oxide-coated substrate 2000 may then be covered with photoresist andpatterned to form openings at the locations of the vias 2110-2145. Thesubstrate 2000 may then be etched through the photoresist to remove theoxide 2200 from the backside openings of the vias 2110-2450. Thephotoresist may then be stripped from the substrate 2000. Since theseprocesses are well known in the art, they are not described or depictedfurther.

The rear surface of substrate 2100 may then be covered with a conductivelayer. In some exemplary embodiments, the conductive layer may be aCr/Au multilayer, chosen for the same reasons as multilayers 1900 and2600, and deposited using the same or similar techniques. Alternatively,the conductive layer may be any conductive material having acceptableelectrical and/or thermal transport characteristics. In one exemplaryembodiment, the conductive material may be a multilayer of 15 nmchromium, followed by 800 nm of nickel, and finally 150 nm of gold. Thenickel may give the multilayer better wear and durabilitycharacteristics that the gold alone over the chromium layer, which maybe important as these features are formed on the exterior of theelectrostatic MEMS plate switch 100.

The conductive layer is then covered once more with photoresist, whichis also patterned with features which correspond to pads 2115, 2125,2405 and 2455 on the backside of the device 100. Alternatively, themetal may be deposited through a shadow mask, allowing for thepossibility of thicker layers and eliminating the need for furtherprocessing.

The conductive layer on the rear of the substrate 2000 is then etched orion milled, for example, to remove the conductive layer at the openingsof the photoresist, to form isolated conductive bonding pads 2115, 2125,2405 and 2455. Conductive bonding pad 2115 may provide electrical accessto the contact points 2110 and 2120 of the switch; conductive bondingpad 2125 may provide electrical access to the contact points 2210 and2220 of the switch; conductive bonding pad 2405 may provide electricalaccess to via 2400 and adjacent electrode 2300 of the switch; andconductive bonding pad 2455 may provide the ground signal to the dualsubstrate MEMS electrostatic plate switch. These bonding pads 2115,2125, 2405 and 2455 are shown in the plan view of the back side of thevia substrate in FIG. 14. After formation of bonding pads 2115, 2125,2405 and 2455, the electrostatic MEMS plate switch is essentiallycomplete, and the wafer pair 1000 and 2000 may be sawed and/or diced toseparate the individual electrostatic MEMS plates switches from theadjacent devices formed on the wafers.

FIG. 15 shows an individual dual substrate electrostatic MEMS plateswitch 100 after manufacture and assembly. In its completed state, theshunt bar 1100 on the deformable plate 1300 hangs adjacent to andspanning the electrical contacts 2110 and 2120, and the deformable plate1300 is also adjacent to the metallization plate 2300, as shown inFIG. 1. Upon applying appropriate voltages to vias 2400 and 2450 usingconductive bonding pads 2405 and 2455, respectively, a differentialvoltage forms across the parallel plate capacitor formed by thedeformable plate 1300 and the electrode 2300, drawing the deformableplate 1300 toward the electrode 2300. At it lower point of travel orvibration of the deformable plate 1300, the shunt bar 1110 affixed tothe deformable plate 1300 is applied across the electrical contacts 2110and 2120 of the switch 100, and shunt bar 1120 is applied acrosselectrical contacts 2210 and 2220, thereby closing the switch. An inputelectrical signal applied to one of the electrical contacts 2110 and2210 by conductive bonding pad 2115 may then be obtained as an outputelectrical signal from either of the other contacts 2120 or 2220 by theother conductive bonding pad 2125. The switch may be opened bydiscontinuing the voltages applied to the plate 1300 and electrode 2300,whereupon the switch may return to its original position because of therestoring spring force acting on the stiff spring beams 1330 coupled tothe deformable plate 1300.

Exemplary thicknesses of various layers of the dual substrateelectrostatic MEMS plate switch 100 are shown in FIG. 16. It should beunderstood that the features depicted in FIG. 16 may not necessarily bedrawn to scale. As shown in FIG. 16, an exemplary thickness of the Cr/Auconductive layer 2600 is about 0.75 μm. An exemplary distance h betweenthe upper surface of the shunt bar 1100 and the lower surface of thecontact point 2112, also defined as the throw of the switch, may be, forexample, about 1.0 μm. An exemplary thickness of the conductive materialof the shunt bar 1100 and contacts 2122 and 2112 may be, for example,about 0.75 μm each. An exemplary thickness of the deformable plate 1300may be about 5.0 μm, which may also be the thickness of the device layer1010. An exemplary thickness of the isolation layer 1200 may be about0.3 μm. Finally, an exemplary thickness t1 of the polymer standoff 2520may be about 1.0 μm, which also defines a minimum separation between theplate substrate 1000 and the via substrate 2000, of the dual substrateelectrostatic MEMS plate switch 100. An exemplary thickness t2 of thealloy bond (In material as well as Cr/Au multilayers) may be about 1.7μm. It should be understood that the dimensions set forth here areexemplary only, and that other dimensions may be chosen depending on therequirements of the application.

While various details have been described in conjunction with theexemplary implementations outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent upon reviewing the foregoing disclosure. For example, while thedisclosure describes a number of fabrication steps and exemplarythicknesses for the layers included in the MEMS switch, it should beunderstood that these details are exemplary only, and that the systemsand methods disclosed here may be applied to any number of alternativeMEMS or non-MEMS devices. Furthermore, although the embodiment describedherein pertains primarily to an electrical switch, it should beunderstood that various other devices may be used with the systems andmethods described herein, including actuators and valves, for example.Accordingly, the exemplary implementations set forth above, are intendedto be illustrative, not limiting.

1. A method for manufacturing an electrostatic MEMS device, comprising:forming a first plate suspended adjacent to a first substrate, whereinthe first plate is coupled to the first substrate by at least one springbeam; forming at least one electrical contact on a second substrate,wherein the first plate is configured to move toward the at least oneelectrical contact; and coupling the first substrate to the secondsubstrate with a seal that seals the MEMS device.
 2. The method of claim1, further comprising: forming at least one shunt bar on the firstplate, disposed substantially on a nodal line of a vibrational mode ofthe first plate; and forming an electrostatic second plate on the secondsubstrate, adjacent to the first plate.
 3. The method of claim 2,wherein forming the electrostatic second plate and the at least oneelectrical contact further comprises: depositing a seed layer over thesecond substrate; depositing a conductive material over the seed layer;and patterning the electrostatic second plate and the at least oneelectrical contact from the conductive material.
 4. The method of claim1, further comprising: forming at least one electrical via through athickness of the second substrate, and electrically coupling the atleast one electrical via to the at least one electrical contact.
 5. Themethod of claim 4, wherein forming the at least one electrical viacomprises: forming at least one blind hole with a dead end wall on afront side of the second substrate; forming a seed layer in the at leastone blind hole; depositing a conductive material onto the seed layer;and removing material from a rear side of the second substrate to removethe dead-end wall of the at least one blind hole.
 6. The method of claim5, wherein depositing a conductive material onto the seed layercomprises plating copper onto the seed layer.
 7. The method of claim 1,wherein coupling the first substrate to the second substrate with a sealcomprises: depositing a first metal between the first substrate and thesecond substrate; and depositing a second metal between the firstsubstrate and the second substrate; and coupling the first substrate tothe second substrate by heating the first metal and the second metal toat least a melting point of at least one of the first metal and thesecond metal, to form a substantially hermetic alloy seal around theMEMS device.
 8. The method of claim 1, wherein the first substratecomprises a silicon-on-insulator substrate, and the second substratecomprises at least one of a silicon wafer and a silicon-on-insulatorsubstrate.
 9. The method of claim 8, wherein forming the first platesuspended over the first substrate comprises: etching a plurality ofholes into a device layer of the silicon-on-insulator substrate; etchinga dielectric layer beneath the device layer of the silicon-on-insulatorsubstrate through the plurality of holes; and etching an outline of thefirst plate in the device layer of the silicon-on-insulator substrate.10. An electrostatic MEMS device, comprising: a first plate suspendedadjacent to a first substrate and coupled to the first substrate by atleast one spring beam; at least one electrical contact formed on asecond substrate, wherein the first plate is configured to move towardthe at least one electrical contact; and a seal which couples the firstsubstrate to the second substrate, and seals the MEMS switch.
 11. Theelectrostatic MEMS device of claim 10, wherein the at least one springbeam comprises at least two spring beams, at least one of the two springbeams disposed on one side of the first plate, and at least one other ofthe at least two spring beams disposed on an opposite side of the firstplate, wherein each spring beam has a segment extending from the firstplate which is coupled to an adjoining segment by a bend.
 12. Theelectrostatic MEMS device of claim 11, wherein the first plate andspring beams are symmetric about at least one of a longitudinal andlatitudinal axis of the first plate.
 13. The electrostatic MEMS deviceof claim 11, wherein the at least one spring beam disposed on one sideof the first plate is anti-symmetric with respect to the at least oneother spring beam disposed on the opposite side of the first plate. 14.The electrostatic MEMS device of claim 10, wherein the first substrateis a silicon-on-insulator substrate including a device layer, a handlelayer and a dielectric layer between the device layer and the handlelayer, and the second substrate is at least one of a silicon substrateand a silicon-on-insulator substrate.
 15. The electrostatic MEMS deviceof claim 10, further comprising: electrical vias formed through athickness of the second substrate; and an electrostatic second plateformed on the second substrate.
 16. The electrostatic MEMS device ofclaim 10, further comprising: a plurality of holes formed through athickness of the first plate, wherein the plurality of holes is disposedin at least one of the following ways: substantially along a latitudinalaxis of the first plate, and in a close-packed hexagonal array.
 17. Theelectrostatic MEMS device of claim 10, wherein the seal comprises: agold/indium alloy which bonds the first substrate to the secondsubstrate with a substantially hermetic seal around the MEMS device. 18.The electrostatic MEMS device of claim 10, wherein the first plate hasat least one shunt bar, located substantially on a nodal line of avibrational mode of the first plate.
 19. The electrostatic MEMS deviceof claim 18, wherein the first plate has two shunt bars located eachlocated substantially along one of two nodal lines of a vibrational modeof the first plate.
 20. The electrostatic MEMS device of claim 19,wherein each shunt bar electrically connects two electrical contactsformed on the second substrate when the electrostatic MEMS switch isclosed, wherein each shunt bar is electrically isolated from otherportions of the first plate.
 21. A method of operating the electrostaticMEMS device of claim 20, comprising: applying a first voltage to thefirst plate formed on the first substrate; applying a second voltage tothe electrostatic second plate formed on the second substrate; formingan electrical connection between the two electrical-contacts by bendingthe first plate and shunt bars toward the electrical contacts inresponse to the applied voltages.
 22. The method of claim 21, furthercomprising: applying an input signal to one of the two contacts formedon the second substrate; and obtaining an output signal from the otherof the two electrical contact formed on the second substrate.
 23. Anapparatus for manufacturing an electrostatic MEMS device, comprising:means for forming a first plate suspended over a first substrate, whichis coupled to the first substrate by at least one spring beam; means forforming at least one electrical contact on a second substrate, whereinthe first plate is configured to move toward the at least one electricalcontact; and means for coupling the first substrate to the secondsubstrate with a seal that seals the MEMS device.